Air Jet Sieving

Air jet sieving is an efficient and effective method for drying fine bulk materials.

Figure 1. Sample of ZrO2 powder sieved at 20 rpm.

Air jet sieving is often the method of choice for the dry sieving of materials with particle sizes below 40 microns. However, it is also a faster alternative to the vibratory sieving of materials of up to 250 microns.

Figure 2. Sample of ZrO2 powder sieved at 55 rpm.

Principles of Operation

In contrast to other methods, air jet sieving is usually carried out with only one sieve. The sieve is placed on the unit with the sample material and covered with a lid. A powerful industrial vacuum cleaner generates a strong jet of air, which disperses the particles on the sieve through the slotted nozzle rotating below the sieve mesh. With each rotation, the particles are dispersed and distributed over the complete sieve surface.

The jet of air causes a continuous new orientation of the particles on the sieve surface. Particles with sizes smaller than the sieve apertures are sucked in by the vacuum cleaner or-if the optional cyclone is used-collected in a laboratory bottle. When using sieves 25 mm high, the inflowing air causes the particles to impact on the lid, which helps to destroy agglomerates.

The nozzle rotating below the sieve usually has a fixed speed, but varying this speed can be helpful. Sensitive sample materials, for example, should be sieved with a low speed to minimize stress on the material. This stress can be further reduced by using a 50-mm-high sieve so that the impact of the sample against the lid is less forceful.

For samples that tend to agglomerate, sieving with high speed is more effective. The impact frequency of the particles against the lid is substantially increased, and even strong agglomerates are dissolved after short sieving times.

Figure 3. The open mesh function enables the nozzle to first move forward by 20° and then backward by 10°, instead of rotating uniformly.

Application Example

Two identical samples of ZrO2 powder were sieved at different speeds (see Figures 1 and 2).* Parameters included:

Figures 1 and 2 clearly show that a high speed is beneficial to break down agglomerates. It is also possible to regulate the negative pressure generated by the vacuum cleaner, which allows variation of the impact speed of the particles. This, together with the selection of the sieve height, greatly improves the sieving of sensitive materials.

Figure 4. Gauze of a 45-micron sieve after sieving quartz sand without (left) and with (right) the open mesh function.

Reproducibility and Performance

Fine-meshed sieves are particularly susceptible to so-called near-mesh particles, which block the sieve gauze. This has a negative effect on the sieve results and can also lead to premature wear of the sieve.

An open mesh function (OMF) can help maintain the performance of the sieve and subsequently improve the reproducibility of results while minimizing time and effort for cleaning. This function enables the nozzle to move according to the principle “two steps forward, one step back,” which means the nozzle first moves forward by 20° and then backward by 10°, instead of rotating uniformly (see Figure 3). Near-mesh particles are therefore blown very effectively from the gauze, as no material lying on the sieve surface obstructs the air jet.

Figure 4 shows the gauze of a 45-micron sieve after sieving quartz sand with and without the OMF. Some sieve apertures are blocked with particles where the sieving was done without the OMF, while the sieve apertures where the OMF was activated are completely free. By gently cleaning the sieve, this special feature helps to improve the reproducibility of the sieve analysis and increase the lifetime of the sieves.

Particle Size Distributions

Although only one sieve can be used with the air jet sieving unit, it is still possible to determine particle size distributions. Two different methods can be used (see Figure 5). In the standard method, the complete amount of material to be sieved is placed on the sieve with the finest mesh size. After sieving and weighing the fraction, the oversize is placed on the next larger sieve and sieved again. This procedure is continued until the complete sample is separated into fractions.

Alternatively, the “Swiss” method requires that the sample is first divided into the number of size classes to be determined. Each part is then sieved individually with the corresponding sieve. This method can only provide reliable results if the sample division is carried out representatively to keep the particle size distribution in all part samples identical. The best results are obtained by rotary sample dividers, which divide the initial material into six, eight or 10 identical part samples.

For the standard method, less sample material is required, whereas the Swiss method is more exact. Software-based evaluation not only facilitates and accelerates the data processing, but also eliminates user errors during data transfer and calculation. The software guides the user through the entire sieving process. It controls the sieve shaker and reads the weighing data to generate a protocol in accordance with standards.

Field Experience

The German company Alpha Ceramics GmbH uses the air jet sieving machine AS 200 for the production of spray-granulated pressed powders from technical ceramics. Alpha Ceramics develops and produces well-engineered materials and products for spray granulation, pressing and fast-firing technologies.

When pressed granules are compacted to components, it is crucial that the particle size distribution of the molding compound contains as little dust as possible. Therefore, spherical granules are produced from one or several raw materials with a particle size of below 2 microns by spray granulation. These granules allow the air to escape during the compaction process and can be automatically filled into the mold due to their flowability (i.e., they are “pourable”).

Insufficient venting during compacting causes compression of the enclosed air, which expands when the pressed product is ejected from the mold and causes fissures in the product. Although Alpha Ceramics has been conducting particle size analyses of the finished spray granules for years, the interpretation of the ascertained particle size range has repeatedly caused the company to consider alternative methods to determine the “dust content” of the pressed granules.

The operation of the analysis instrument was simplified, along with the interpretation of the results, with the introduction of the air jet sieving machine for quality control. If 50 g of material is sieved with a 45 µm sieve for 8 minutes with open mesh function, at least 49 g must remain on the sieve (which corresponds to a maximum of 2% dust content). If the percentage of fine particles is higher, the spray drying process parameters need to be corrected.

“The introduction of this quality control measure proved to be uncomplicated and effective and, not least due to the easy handling of the instrument, was immediately accepted by all employees,” said Robert Kremer, Alpha Ceramics’ technical director.

Ensuring Quality

Air jet sieving is an important method of quality control for fine bulk goods. Modern technologies and the possible variations of sieving parameters, in combination with reliable evaluation software, ensure reproducible sieving results over a long period of time.